The present invention relates to reactive thermally-curing and/or radiation-curing paints and their curing methods.
Most metallic and many plastic surfaces of articles for use are painted to prevent weathering and corrosion and for decorative reasons. This holds especially true in the automotive industry. Two-component polyurethane paints (2C-PUR) have become a standard technology since they cure at low temperatures and can be adjusted to flexibly adapt to the substrate. In addition, these systems are distinguished by satisfactory weather resistance.
Light-curing paints in particular assure qualitative improvements. Of these paints, UV-cured paints are the main ones that are used. Typical fields of application for such paint systems can be found in the electronics industry, printing, furniture, parquet flooring and paper industry.
Such paints contain binders having ethylenically-unsaturated groups which are suitable for polymerization (reactive groups). Frequently, acrylate or methacrylate groups are used as the reactive groups. The hardening reaction that can be started by photoinitiators of known paint systems is usually radical polymerization triggered by UV radiation.
Multilayer automobile coatings generally consist of a sequence of several functional layers that contain an anticorrosion layer such as a phosphate coating, primer, a pigment-containing base coat and a final transparent clear coat.
The top paint layers generally consist of:                a base coat for providing color and setting the color effect (such as metallic), and        a clear coat as a protective layer against weathering, solvents and scratches.        
The mechanical properties of the light-curing or UV-curing paint systems typically are better than those of conventional annealed enamels. It is very difficult to impossible to completely cure UV paints in shaded areas. This is particularly true of geometrically complex components where, depending on the application method such as dipping, spraying or spattering, the paint is applied or deposited in shaded areas.
Curable clear coats with binders based on poly(meth)acrylic functional compounds, polyisocyanate, light stabilizers and photoinitiators are known from European Patent Application No. 247 563 A1. The final hardness is achieved by additional cross-linking in which a part of the light-cross-linkable binders react via free hydroxyl groups with polyisocyanates to form urethanes. This combination of acrylate-containing monomers and acrylate-free monomers is disadvantageous to the paint quality.
European Patent Application No. 540 884 A1 includes a prior-art two-step method for producing a multilayer coating for automobiles. The paint formulation that is used in a final clear coat contains radically and/or cationically-curing binders. The hardening is started by UV radiation, and the final hardness is only attained in a second step. The second step is preferably annealing at temperatures of approximately 80–160° C. In one of the variations of the method, heat-activatable radical initiators are also used.
The known paint systems generally cannot meet the demands of high-quality automobile paints, or they cannot satisfactorily solve the problem of complete hardening in shaded areas. In particular, the paint quality is insufficient for hardening under air.